Hi Rebecca,
Did you check the time zone in php info in Site administration > Server? Have you in your php.ini file a line with "date.timezone"?
Are you in a time zone without daylight saving time? The reason why I ask this is that I suspect Moodle 2.8.1 to apply daylight saving time to all time zones regardless of what you select in "Forced timezone" except for MST7MDT or PST8PDT or EST5EDT or CST6CDT. The "DT" in the end of the time zone ensures that daylight saving is not applied. The one hour daylight saving can than explain the different start date when you manually enroll users.
With kind regards,
Hartmut